It has been reported in the literature that the transitive subject control use of the verb promise, as illustrated in John promised Mary to leave, is subject to considerable interspeaker variation. In this chapter, I argue that this variation manifests itself because of input indeterminacy: the post-verbal goal expression may be structurally represented as PP (Grammar A) and KP (Grammar B). Both grammars are consistent with the incoming primary linguistic data and are available within the narrowly specified hypothesis space constrained by Universal Grammar. This input indeterminacy theory predicts that the PP vs. KP ambiguity, a case of syntactic vulnerability, should not be restricted to the promise-construction but instead should be found more generally. I show that this prediction is borne out in three arenas: Japanese case-marker/postposition -ni, diachronic relationships between PP-ergative and DP-ergative languages such as Niuean and Adyghe, and the ambiguous categorial status of experiencers in experiencer-object psych verb constructions in languages such as Greek, English, and Italian.